Power Analyses for an Unconditional Growth Model using {lmer}

Recently, I was asked to knock together a quick power analysis for a linear growth model with approximately 120 subjects. Having already collected data (i.e., having a fixed sample size), the goal of the power analysis was to explore whether a sample of 120 subjects would be sufficient to detect significant linear change (\(\alpha = .05\)) for a secondary research question that was not part of the original proposal (we added collection of a set of variables partway through the data collection period).

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